Measuring process for obtaining a switching command in a rapid changeover device

ABSTRACT

A measuring process for obtaining a switching command in a rapid changeover device in which an abrupt change in the voltage in a consumer device is detected and a voltage difference is formed using the voltage of a power supply source to be connected and the voltage at the consumer device, and the switching command for a power switch is issued taking its response time into account if, at the time of the changeover, a voltage difference with a value below a permissible maximum is pre-calculated. To perform such a measuring process as accurately and as quickly as possible, the peak values and the frequencies and the phase relation of the voltages in the first and second power supply sources are found when disconnecting the consumer device. After the disconnection of the consumer device, the time change of the frequency and the time change of the magnitude of the voltage at the consumer device are found. These values are then used to calculate the moment of changeover.

BACKGROUND OF THE INVENTION

The present invention relates to a process for obtaining a switchingcommand in a rapid changeover device for switching over a consumerinstallation, as needed, from a first power supply source to a secondpower supply source. After the consumer installation is disconnectedfrom the one power source, a differential voltage is formed in theconsumer installation from the voltage of the second power supply sourceand from the voltage at the consumer installation. The switching commandis then given, in view of the inherent response time of the power switcheffecting the changeover, when a differential voltage is pre-calculatedfor the moment of changeover with a value below a permissible maximumvalue.

A process of this type can be inferred from the German PatentPublication No. 21 08 307 C3 (hereinafter "the 307 publication"). In aprocess such as the one inferred from the '307 publication, an auxiliaryvoltage proportional to the mean differential voltage is formed from thevoltage of the power supply source to be connected at any one time andfrom the voltage at the consumer installation. Additional auxiliaryvoltages are formed from this auxiliary voltage through repeateddifferentiation, so that the auxiliary voltages represent single termsof a series expansion, preferably of a Taylor series. The auxiliaryvoltages are generated proportionally to the product from the derivativeof the particular order and to a quantity of a corresponding power, thequantity representing a selectable time factor. The thus formedauxiliary voltages are added and compared to a specified voltage toobtain a switching command. If the selectable time factor corresponds tothe inherent response time of the switch effecting the changeover, thenthe known process allows a changeover to be produced in view of theinherent response time of the particular switch, at an instant when thesum of the auxiliary voltages falls below a specified value, so that thechangeover is substantially bumpless (i.e., seamless).

Rapid changeover devices safeguard the power supply of importantconsumer installations, particularly for motor configurations. Rapidchangeover devices are essential for auxiliary switchgear used in powerstations, for which at least two power supply sources are almost alwaysprovided, the power supply sources being as independent as possible fromone another. In such auxiliary switchgear, all machines and unitsrequired for the operation are supplied from a station-service bus of agenerator block. During start-up of the generator block, the requiredpower is obtained from the power supply system. After the start-upoperation is complete, the internal power consumption requirement isobtained from the block itself.

In the case of a malfunction (for example a short-circuit, a drop infrequency or voltage caused by an overload, etc.) in the supply systemor the generator, a secondary power supply source must be switched towithin a few seconds, to continue to safeguard the internal powerconsumption of the power station. During such a changeover operation,the switch to the connected power supply source is first opened, therebydisconnecting the consumer installation from the incoming power supply.As a result, the voltage at the consumer installation decreases withrespect to frequency and amount with a time constant specific to theinstallation. After the consumer installation is disconnected from theone power supply source, the changeover to the secondary power supplysource must be made as quickly as possible. However, this can onlyhappen when the differential voltage between the secondary power supplysource to be connected to the system and the consumer installation liesbelow an adjustable value. In this case, the inherent response time ofthe power switch effecting the changeover at any one time must beaccounted for.

Starting from the known measuring process described above, the object ofthe present invention is to provide a measuring process for obtaining aswitching command in a rapid changeover device, which will allow thedifferential voltage to be calculated with relative accuracy, in view ofthe inherent response time of the power switch effecting the changeoverat any one time, so that the changeover from the first power supplysource to the secondary power supply source will be bumpless (i.e.,seamless) to the greatest possible extent.

SUMMARY OF THE INVENTION

The present invention solves the above mentioned objective in the caseof a measuring process of the type indicated at the outset bydetermining the peak values and the frequencies of the voltages at theconsumer installation and at the second power supply source, and thephase relation of the voltage at the second power supply source relativeto the phase relation of the voltage at the consumer installation whenthe consumer installation is disconnected. The resulting abrupt changein the voltage at the consumer installation is acquired. After theconsumer installation is disconnected, the change in the frequency andthe change in the amount of the voltage at the consumer installation aredetermined as a function of time. Subsequently, to calculate the instantof the changeover, the differential voltage defined by the expression:

    U.sub.2max -(1-c)·(l-b·t.sub.s)U.sub.3max exp(j(ω.sub.2 -ω.sub.3) t.sub.s -0.5·a·t.sub.s.sup.2 +Γ.sub.2 -Γ.sub.1)

where

ω₂ =2· f₂ and

ω₃ =2 f₃, is evaluated.

The important advantage of the measuring process according to thepresent invention is the ability to determine the amount of thedifferential voltage, while fully allowing for the magnitudes and thephase relations, as well as the frequencies of the voltage from thesecondary power supply source, and of the voltage at the consumerinstallation, so that by simultaneously considering the inherentresponse time of the power switch effecting the changeover, a switchingcommand can become effective when the differential voltage lies below acomparatively low specified value. As a result, a substantially bumpless(i.e., seamless) changeover is achieved.

In the measuring process according to the present invention, the changein the frequency of the voltage at the consumer installation can bedetermined as a function of time in a novel way to more simply implementthe process and to achieve the greatest possible accuracy and speed.After the consumer installation is disconnected, the voltage at theconsumer installation is sampled with a sampling frequency being amultiple of the frequency of the voltage at the consumer installation.When measuring signals, corresponding to the sine and cosine componentof the fundamental wave of the voltage at the consumer installation areacquired from the sampled values at a sampling instant in a Fourierfilter, the phase relation of the corresponding complex measurablequantity is determined using the measuring signals. The prevailingfrequency of the voltage at the consumer installation is determined fromthe difference between the phase relation of another complex measurablequantity acquired at a corresponding sampling instant in a followinghalf wave of the voltage at the consumer installation and the phaserelation of the one complex measurable quantity according to therelation:

    f.sub.3meas,i+1 =(ΔΦ·f.sub.3meas,i)/

where f_(3meas),i indicates the frequency of the voltage at the consumerinstallation determined in the preceding measuring cycle.

The process for measuring the change in the frequency of a voltage canalso be advantageously applied by itself.

The change in the amount of voltage at the consumer installation afterit is disconnected from the one power supply source can usually bedetermined by making a voltage comparison in equidistant time intervals.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a power plant installation including a rapid changeoverdevice which operates according to the process of the present invention.

FIG. 2 is a diagram which clarifies the process for measuring a changein frequency.

DETAILED DESCRIPTION

As FIG. I reveals, a rapid changeover device S is connected via avoltage transformer W₁ to a supply line SL₁ (voltage U₁). Through thesupply line SL₁, a consumer installation V is supplied from a powersupply source E₁ by means of a power switch S₁. The consumerinstallation V contains a station service bus ES and, for example, amotor M. The consumer installation V may be linked via a second powerswitch S₂ and a second power supply line SL₂ to a second power supplysource E₂. A voltage corresponding to the voltage U₂ on the second powersupply line SL₂ can be acquired by the rapid changeover device S viasecond voltage transformer W₂. Finally, the voltage U₃ at the consumerinstallation V can be acquired by the rapid changeover device S via athird voltage transformer W₃.

To further clarify the process according to the present invention, inthe following example it is assumed that a changeover should be madefrom the one power supply source E₁ to the second power supply sourceE₂, because the power supply source E₁ is experiencing fault conditions.

In the assumed case, before the power switch S₁ is actuated, the voltageU₃ at the consumer installation V (or at the station service bus ES) isequal to the voltage U₁ on the power supply line SL₁. Therefore, in aninstantaneous value representation:

    u.sub.3 =u.sub.1 =u.sub.1max ·sin(ω.sub.1 ·t+Γ.sub.1)                                (1)

or in a polar representation:

    U.sub.3 =U.sub.1 =U.sub.1max ·exp(j(ω.sub.1.t+Γ.sub.1))(2)

where,

u_(1max) denotes the peak value of the voltage U₁

ω₁ denotes the angular frequency (2· ·f₁) of the voltage U₁, and

Γ₁ denotes the phase angle of the voltage U₁.

Accordingly, the voltage of the additional power supply source E₂ to beconnected to the system can be expressed by the following relations:

    u.sub.2 =u.sub.2max ·sin(ω.sub.2 t+Γ.sub.2);(3)

or

    U.sub.2 =U.sub.2max ·exp(j(ω.sub.2 t+Γ.sub.2)), (4)

where,

u_(2max) denotes the peak value of the voltage U₂

ω₂ denotes the angular frequency (2· ·f₂) of the voltage U₂, and

Γ₂ denotes the phase angle of the voltage U₂.

At the instant the consumer installation V is disconnected from the onepower supply source E₁ (i.e., when the power switch S₁ is opened) thevoltage U₃ at the consumer installation V experiences an abrupt changewhich can be expressed by the following equation (5)

    U.sub.3 =c·U.sub.1.                               (5)

After the consumer installation V is disconnected from the one powersupply source E₁, speed of rotation of the motor M decelerates due tofriction losses. The retarding torque can be regarded as the averagetorque of all attached drives, and leads to a temporal change in thefrequency in the sense of a reduction in frequency. This has beendescribed in the '307 publication. (See column 3, paragraph 2). Thetemporal change in the frequency occurs linearly over time, so that theangular frequency ω₃ of the voltage U₃ at the consumer installation Vcan be described by the following relation (6):

    ω.sub.e =ω.sub.1 -∝t

where,

∝ denotes a constant indicating the change in the frequency of thevoltage U₃ at the consumer installation V as a function of time, and

t denotes the time that elapsed since opening the power switch S₁.

After the abrupt change has taken place, the voltage U₃ also decreaseslinearly, which can be described by the factor 1-b·t, where b specifiesthe decrease factor of the voltage U₃. Thus, the characteristicwaveshape of the voltage U₃ at the consumer installation V can beindicated in an instantaneous value by the following relation:

    u.sub.3 =u.sub.1max ·(1-c)·(1-c)·(1-bt)·sin(ω.sub.1 t +Γ.sub.1 -0.5·∝·t.sup.2).(7)

The polar representation of the instantaneous voltage U₃ at the consumerinstallation can be determined by the following equation:

    U.sub.3 =U.sub.1max ·(1-c)·(1-bt)·exp(j(ω.sub.1 ·t+Γ.sub.1 -0.5·∝·t.sup.2)).(8)

As explained above, at the instant the additional power switch S₂ isswitched on or closed, the differential voltage ΔU formed from thevoltage U₂ of the additional power supply source and from the voltage U₃at the consumer installation must be smaller than a predetermined valueΔU_(permissible). This can be expressed by the following relation (9):

    ΔU.sub.3 =|U.sub.2 -U.sub.3 |≦ΔU.sub.permissible.               (9)

Therefore:

    ΔU=|U.sub.2max -(1-c)·(1-bt.sub.2)U.sub.1max exp(j(ω.sub.2 -ω.sub.1) t.sub.2 -0.5·∝·t.sub.s.sup.2 +Γ.sub.21))|≦ΔU.sub.permissible(10)

This demonstrates that the measuring process according to the presentinvention can be carried out to obtain a switching command when allquantities of the relation (10) indicated above have been determined.

No difficulties arise with respect to determining the quantitiesU_(2max) or U_(1max), because these quantities can be measured once andretained before the changeover process. The rapid change c in thevoltage U₃ can likewise be measured and stored during the disconnectoperation.

The change b in the voltage U₃ is easily determined as a function oftime after the consumer installation V is disconnected from the onepower supply source E₂ by making voltage comparisons in equal timeintervals and is likewise stored as a fixed quantity. Also, nodifficulties are entailed in determining the angular frequencies ω₂ andω₁ before the beginning of the changeover process and these measuredquantities are also retained. The quantity t_(s) describes the inherentresponse time of the power switch SL₁ or SL₂ being used and is a knownquantity.

Also determining the phase angle Γ₂ -Γ₁ between the voltages U₂ and U₁immediately before the changeover operation does not entail anydifficulties. This phase angle is also stored.

Thus, in light of the equation indicated above, to obtain a switchingcommand, only the quantity ∝, which describes the change in thefrequency of the voltage U₃ as a function of time after the consumerinstallations V have been disconnected from the power supply source E₁needs to be defined. The process defines the quantity by:

Sampling the voltage U₃ with a sampling frequency f_(A), which is amultiple of the frequency f₃ of the voltage U₃. N sampling instantsresult then for each period of the voltage U₃. The frequency to bemeasured results then as:

    f.sub.meas =N·f.sub.A.                            (11)

A phasor can be formed from the sampled values of the voltage U₃ in thecomplex plane, in that the individual sampled values are supplied to aFourier filter. In this Fourier filter, the sine and cosine component ofthe fundamental wave are acquired. This process is advantageous since itis insensitive to harmonic oscillations and eliminates the steady (i.e.,zero-frequency, D.C.) component, for example the sampling offset. Forexample, if twelve samples a(k) are made for each period of the voltageU₃, and if a sampling frequency of f_(A) =600 Hz is used, then thefollowing filter specification results, in that sampling values of oneperiod of the system frequency U₃ are summed with correction factors.The following summation results for the sine component s(k) at theoutput of the sine filter:

    s(k)=0.5·a(k-1)+7/8·a(k-2) +a(k-3)+7/8·a(k-4)+0.5·a(k-5)-0.5·a(k-7)-7/8.multidot.a(k-8)-a(k-9)-7/8·a(k-10)-0b 0.5·a(k-11)(12)

The following series (summation and subtraction) results for the cosinecomponent c(k):

    c(k)=a(k)+7/8·a(k-1)+0.5·a(k-2) -0.5·a(k=4)-7/8·a(k-5) -a(k-6)-7/8·a(k-7)-0.5·a(k-8)+0.5·a(k-8)+0.5.multidot.a(k-10)+7/8·a(k-11)                        (13)

In each of these series, a(k-n) describes the sampling value, which liestimewise by one value n before the current sampling value a(k). Theoutput signals from the sine and cosine filter are in phase quadrature.A vector r(k), whose coordinates are described by c(k) and s(k), resultsin the complex plane (see FIG. 2). In the time at, this vector r(k)covers an angular difference of ΔΦ. The frequency f_(meas),i+1 of thesampled signal can be easily calculated from this as:

    f.sub.meas,i+1 =ΔΦ/(2· ·Δt).(14)

To calculate the prevailing frequency using this equation, it isnecessary to define Δt. This quantity can be expressed by the samplingfrequency f_(A) and by the number of the sampled values N for eachperiod of the voltage U₃. The angular difference is developed from theposition of two vector positions, which are identically situated withrespect to the sampling instants in successive half waves of the voltageU₃. Given a system frequency of 50 Hz, the position of two vectorpositions, which are spaced apart by 10 ms is preferably used. Given asampling frequency of f_(A) =600 Hz, this 10 ms time corresponds to N=6sampled values or to a rotational angle of ΔΦ=180°. The samplingfrequency f_(A) =600 Hz leads to one complete revolution of the vectorin 20 ms, when the signal frequency f_(meas),i corresponds to the systemfrequency of 50 Hz. Thus, Δt can be described by the following relation(15) as:

    Δt=1/f.sub.A ·N/2=1/(2·f.sub.meas,i)(15)

In equation (15)--as already mentioned above--the calculation is made atthe beginning of the measurement of the change in frequency withf_(meas),i =50 Hz as a basis.

Thus, the prevailing frequency of the voltage U₃ at the consumerinstallation V can then be expressed from equations (14) and (15)through the following relation (16) as:

    f.sub.meas,i+1 =(ΔΦ·f.sub.meas,i)/      (16)

In this manner, measurements of the current frequency of the voltage U₃can be made one after the other in the same time intervals. The changein the frequency ∝ can then be simply calculated as a function of timefrom these measurements through the following relation (17):

    ∝=(f.sub.meas,i -f.sub.meas,i+1)/Δt           (17)

All quantities in equation (10) indicated above are then determined, andthe prevailing differential voltage ΔU can then be ascertainedarithmetically. If the prevailing differential voltage ΔU falls below aspecified, permissible, largest value of ΔU_(permissible), then aswitching command is given to the power switch S₂, causing it to connectthe consumer installation V to the additional power supply source at aninstant in which the differential voltage ΔU is smaller than thepermissible value.

With respect to defining the quantity ∝ (or with respect to measuringthe change in the frequency of the voltage U₃), given a larger deviationin the frequency of this voltage from the system frequency Hz, thesampling frequency should be corrected accordingly, to maintain theaccuracy of the process. This correction is realized by altering thesampling frequency, and by storing a new numerical value at any one timein the counter which produces the sampling rate. This numerical valuecan be stored in a table.

I claim:
 1. A process for issuing a switching command in a rapidchangeover device for switching a consumer installation from a powersupply source to a second power supply source by means of a power switchhaving an inherent response time t_(s), the process comprising stepsof:a) disconnecting the consumer installation from the power supplysource; b) when disconnecting the consumer installation from the powersupply source,(i) measuring a peak value U_(3max) and a frequency f₃ ofthe voltage U₃ at the consumer installation, a peak value U_(2max) and afrequency f₂ of the voltage U₂ at the second power supply source, and arelation between a phase angle Γ₃ of the voltage at the consumerinstallation and a phase angle Γ₂ of the voltage at the second powersupply source; (ii) storing the peak value U_(3max) and the frequency f₃of the voltage U₃ at the consumer installation, the peak value U_(2max)and the frequency f₂ of the voltage U₂ at the second power supplysource, and the relation between a phase angle Γ₃ of the voltage at theconsumer installation and a phase angle Γ₂ of the voltage at the secondpower supply source; (iii) measuring and storing an abrupt changecU_(3max) in the voltage U₃ at the consumer installation; c) after theconsumer installation is disconnected from the power supply source,measuring a change (a) in frequency f₃ and a change (b) in voltage U₃ atthe consumer installation as a function of time; d) subsequently forminga differential voltage signal ΔU from the voltage of the second powersupply source U₂ and from the voltage at the consumer installation U₃,the differential voltage ΔU being defined by the expression:

    |U.sub.2max -(1-c)·(1-b·t.sub.2)U.sub.3max exp(j(ω.sub.2 -ω.sub.3)t.sub.2 -0.5 at.sub.s.sup.2 +Γ.sub.2 -Γ.sub.1)|

where, ω₂ =2· f₂ and ω₃ =2 f₃ ; e) issuing a switch command to effect aswitchover such that, in view of the inherent response time t_(s) of thepower switch, the switchover will occur when the differential voltage ΔUformed in step (f) is below a permissible maximum value.
 2. Themeasuring process according to claim 1 wherein said step of measuringthe frequency at the consumer installation comprises the sub-steps of:i)sampling the voltage at the consumer installation U₃ with a samplingfrequency f_(A), the sampling frequency f_(A) amounts to a multiple ofthe frequency f₃ of the voltage U₃ at the consumer installation;ii)acquiring measuring signals from the sampled values at a samplinginstant in a Fourier filter, the measuring signals correspond to sineand cosine component of a fundamental wave of the voltage at theconsumer installation U₃ ; iii) measuring a phase relation of acorresponding complex measurable quantity with the measuring signalsacquired in sub-step (ii); and iv) measuring the prevailing frequency ofthe voltage at the consumer installation from the difference ΔΦ betweenthe phase relation of another complex measurable quantity at acorresponding sampling instant in a following half wave of the voltageat the consumer installation U₃ and the phase relation of the onecomplex measurable quantity, according to the relation:

    f.sub.meas,i+1 =(ΔΦ·f.sub.3meas,i)/ ,

where f_(3meas),i indicates the frequency of the voltage U₃ at theconsumer installation measured in the preceding measuring cycle.